#define _CRT_SECURE_NO_WARNINGS 1
/**
*      author: Y_Yao
*      created: 2025-02-08 21:24:45
**/

#include<bits/stdc++.h>

#define int LL
using namespace std;
using LL = long long;
const LL LLINF = 1e18;
const int INF = 0x3f3f3f3f;
const int N = 1e6 + 7;
int n;
int a[N];
unordered_set<int> s;

void solve()
{
    cin >> n;
    for (int i = 1;i <= n;i++)
    {
        cin >> a[i];
        s.insert(2 * a[i]);
    }

    sort(a + 1, a + 1 + n);
    int ans = 0;
    for (int i = 1;i <= n;i++)
    {
        int target = 2 * a[i];
        int l = i + 1, r = n;
        while (l < r)
        {
            if (a[l] + a[r] > target)
            {
                r--;
                continue;
            }
            if (a[l] + a[r] < target)
            {
                l++;
                continue;
            }
            if (a[l] + a[r] == target) ans++;
        }
    }
    cout << ans << endl;
}

signed main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);

    int t = 1;
    //cin >> t;
    while (t--)
    {
        solve();
    }
    return 0;
}